The present invention relates to a terminating call control method and a system therefor and, more particularly, a terminating call control method and a system therefor in a digital switching unit incorporating subscriber lines and having a common line signal processing device and a subscriber information storage device.
A prior art will be described with reference to FIG. 2.
FIG. 2 shows a switching system for explaining a conventional terminating, or incoming, call control scheme. This scheme is referred to as a first conventional scheme.
FIG. 2 shows a switching system part constituted by a host switching unit 1 and lower switching units 2 and 3. Subscriber telephone sets 5 and 6 constitute a subscriber group 4 which shares a terminating, or incoming, call. The subscriber telephone sets 5 and 6 are connected to the switching units 2 and 3 through subscriber lines, respectively. The switching units 1, 2, and 3 comprise central processing units 40, 41, and 42, time-division networks 10, 11, and 12, and common line signal processing devices 50, 51, and 52, respectively. The switching unit 1 also has speech path trunks 20 and 21 and signal trunks 30 and 31. The subscriber group sharing a terminating call is defined as a group of subscriber telephone sets in which a call (including both extension and incoming calls) terminated at a given telephone set can be received by another telephone set or transferred thereto. The switching units 2 and 3 also have speech path trunks 22 and 23, signal trunks 32 and 33, subscriber control circuits 60 and 61, and subscriber circuits 70 and 71, respectively. In addition, the switching units 1 to 3 comprise subscriber information storage devices (not shown), respectively. Each subscriber information storage device stores basic information (a subscriber number, accommodation position information, and service information) of each subscriber telephone set accommodated in a home switching unit, and a subscriber group number representing a subscriber group to which each subscriber of the home switching unit belongs, if any subscriber group is present.
The operation of the switching system in FIG. 2 will be described below.
Referring to FIG. 2, when a fault has occurred in the switching unit 3 to disable call processing (a switching unit in which the fault has occurred is referred to as a fault switching unit hereinafter), the speech path trunk 23 of the switching unit 3 is closed. When the speech path trunk 21 of the host switching unit 1 confirms that the speech path trunk 23 is closed, this is informed to the central processing unit 40 through the time-division network 10.
When the switching unit 1 receives a terminating call to the subscriber telephone set 6 connected to the subscriber line accommodated in the fault switching unit 3, the central processing unit 40 informs a switching unit issuing the terminating call of a call incoming disable state because the fault has occurred in the switching unit 3 to disable call processing, thereby completing the processing.
A call control scheme for remedying a terminating call to a subscriber telephone set accommodated in the fault switching unit upon occurrence of the fault in the switching unit to disable call processing is described in Japanese Patent Laid-Open No. 57-155867. This scheme is referred to as a second conventional scheme.
In the second conventional scheme, a telephone set of duplicate homing arrangement connected to both a first subscriber line accommodated in a first switching unit and a second subscriber line accommodated in a second switching unit is arranged, subscriber numbers of duplex number service which consist of a first telephone number sent through the first switching unit and a second telephone number sent through the second switching unit are assigned to this telephone set. A calling switching unit selects a speech path passing through the first switching unit in accordance with the first telephone number of the subscriber numbers. When a fault occurs in the first switching unit or the first subscriber line, the first switching unit sends a communication disable signal to the calling switching unit. Upon reception of the communication disable signal, the calling switching unit reads out the second telephone number from the subscriber numbers to select the speech path passing through the second switching unit.
According to the first conventional scheme, when a fault occurs in a switching unit accommodating a subscriber telephone set constituting the subscriber group to disable call processing, a terminating call to a subscriber telephone set connected to the fault switching unit is disabled. According to the second conventional scheme, two subscriber lines are required for one subscriber telephone set to result in high cost. In addition, the line efficiency is degraded because the two subscriber lines are assigned to one telephone set for a fault which rarely occurs.